DESCRIPTION (adapted from the Abstract): Microsporidiosis is clearly an important group of emerging pathogens. Research on the human pathogen Enterocytozoon bieneusi has been hampered both by a lack of in vivo animal models as well as in vitro cultivation systems. A model of E. bieneusi has been established in which both human and simian isolates of E. bieneusi have been transferred successfully to immunosuppressed gnobiotic (GB) piglets by infection them with spores of E. bieneusi stored for as long as two years at 4 degrees Centigrade. In addition, transmission has been successful from piglet to piglet. The goal of this research is to characterize the immunosuppressed GB piglet model of E. bieneusi for the evaluation of antimicrobial agents against this parasite. The course of infection will be studied, including some fundamental aspects of parasite biology and its interaction with the host cell. Information is lacking due to an absence of animal models. The specific aims are: To characterize E. bieneusi infection in the GB immunosuppressed piglet model. To study the course of infection, clinical manifestation, and pathogenesis. To determine the relationship between immunosuppression and extent of infection to optimize the model. To study early events in host cell pathogen interaction at the ultrastuctural level. To optimize standardize and validate the immunosuppressed GB piglet model for preclinical testing of therapeutic formulations against E. bieneusi with a view of testing five per year for Years 2-5 of the support period. The hypothesis is that the GI tract in piglets and humans have similar immune responses to this infection and that pigs represent a good animal model of this disease. The development of this model is the focus of this application, not basic advances in biology. This model will be of immediate use for drug testing and also as a source of E. bieneusi spores for antibody development (diagnostics) and basic biology. The development of this porcine model would provide an invaluable source of parasitic material for additional investigations and accelerate research on this organism. Given the cost and nature of the simian SIV model it is unlikely this model would serve the same function.